1. Field of the Invention
The present invention relates to a device and method for controlling a prestressing jack when tensioning a tendon.
2. Description of the Background Art
Devices in accordance with the invention are useful in all applications where at least two, but generally many, tension members of a structure are to be tensioned one after the other such that a uniform state of tension prevails in all tension members after the completion of the tensioning process. Cited by way of example are prestressed structures or structural members that, under the compressive preloading introduced during tensioning of the tendons, suffer elastic and plastic deformations, in particular compressions, which, when the tension members are successively tensioned individually or in groups, result in a reduction in the tensioning forces in each of the previously tensioned tension members, and thus result in nonuniform tension states. In order to achieve the result that all tension members have the same tensioning force in the final state, it is thus known to predetermine the sequence of tensioning, and to calculate precisely the tensioning forces to be introduced into each tension member, taking into account the deformation occurring in each case. Since the assumptions to be made for the deformations of the structure, in particular, are often uncertain, this resource-intensive process does not always result in a uniform distribution of tension.
An analogous situation results during the tensioning of free tension members, such as the cables for cable-stayed and suspension bridges. Such cables are generally composed of a plurality of individual tension members, for example steel wires, steel rods, or stranded steel wires, which must be tensioned in order to achieve the operational state. On the one hand, the tensioning force can be applied by tensioning all the individual elements simultaneously, although this proves to be very resource-intensive and cost-intensive on account of the large and correspondingly heavy prestressing jacks that are required here.
It is thus preferred, in contrast, to successively tension the individual tension members, although it is necessary to take into account during this process that the tensioning force of the previously tensioned individual tension members decreases with the tensioning of each [additional] individual tension member. In order to nevertheless obtain a uniform state of tension in all the individual tension members, it is thus necessary for all individual tension members except the last to be over-tensioned by a specific amount characteristic for each individual tension member.
An associated problem also exists in the field of geotechnical engineering, where ground anchors with one or more tension members are anchored within a drilled hole deep in the subsoil and are tensioned against an abutment on the exposed side of the drilled hole. Especially in the case of stepped anchors with tension members of different lengths, simultaneously tensioning all tension members by a uniform tensioning distance results in different states of tension, which is why the individual tension members are adjusted to a uniform load in the operational state through successive tensioning.
In this context, a tension member made of stranded steel wires is known from EP 0 421 862 B1, which corresponds to U.S. Pat. No. 5,083,469, and in which the first tensioned strand is used as a reference strand, and is provided with a force measuring device. Each additional strand is then to be tensioned to the tensioning force that the reference strand has at the time when the additional strand is tensioned. In this procedure, the tensioning force to be applied to the first strand is calculated at a value above the final tensioning force on account of the expected deformations of the structure. Since the tensioning force inherent in the strands that have been tensioned at any point is always the same in this method, the reference strand always reflects the current tensioning force in the individual strands. Consequently, changes in the deformation, for example due to temperature differences, do not affect the uniform state of tension. However, carrying out this method requires complex force measurement devices which must be installed before tensioning, observed during the tensioning process, and removed again afterwards.
A possibility for successive tensioning of tendons or individual elements of tension members that is improved in comparison is known from DE 195 36 701 A1, which corresponds to U.S. Pat. No. 5,809,710. Described there is a tensioning device with first and second prestressing jacks that are connected to one another through a tension line and a return line and that form a hydraulically communicating system. The tensioning of the reference tendon initially takes place solely with the first prestressing jack, bypassing the second prestressing jack. The subsequent tendons are then tensioned with the second prestressing jack until the beginning of a longitudinal motion in the reference tendon resulting from the uniform pressure in both prestressing jacks is discernible, signaling a uniform state of tension in the two tendons. In this way, all tendons can be tensioned gradually to the same tensioning force without the need to make cumbersome measurements.